Nondestructive evaluation (NDE) technologies have been recently challenged to find material defects such as fatigue flaws, cracks and damage precursors such as stress or corrosion induced local conductivity variation in structures with higher probability of detection (POD) and a level of improvement is necessary as these issues are critical to operational safety. Certain inspection opportunities and their specific geometries often necessitate off-the-surface or non-contact methodologies thereby eliminating methods such as ultrasound testing where either physical contact or a transmit medium necessary for inspection. The non-destructive testing industry currently does most non-contact inspection of conductive materials via conventional wound inductive coil based eddy current inspection techniques. These inspections have limited spatial resolution due to sense coil size as well as frequency dependent sensitivity and thus have limited efficacy.
Eddy current testing (ECT) probes to locate and characterize flaws or material defects in a conductive material are known. An ECT probe does this by sensing the out-of-plane magnetic flux leakage (MFL) created by the deviation of eddy currents by the flaws or defects in the area under test (AUT).
Technological advancements in the manufacturing of these elements have led to commercially accessible sensing elements. Low cost anisotropic magnetoresistance AMR and giant magnetoresistive GMR magnetometers (referred to collectively as “XMR” sensors herein) are now available which are sensitive, have small package size, consume little power, and operate at room temperature.
ECT utilizing XMR sensing can have a higher level of utility, as these sensing elements are non-inductive and orders of magnitude smaller than traditional eddy current coils. A magnetoresistive (MR) sensor is a solid-state device that utilizes electron conduction physics to convert a magnetic field into an electrical signal. Anisotropic magnetoresistance (AMR), for example, is a solid-state sensing element that has a permalloy (Ni80—Fe20) electrodeposited line on silicon for sensing low-level magnetic fields. This occurs by an alignment of the material's magnetic domains in response to the external magnetic field of interest. This magnetic domain alignment changes the resistivity of the sensor via induced changes in the scattering matrix (spin-coupled interaction between the conduction electrons and the magnetic moments in the material)). In contrast a magnetic sensor exhibiting the giant magnetoresistive (GMR) mechanism will convert a sensed magnetic field to an electrical signal is exploiting the spin-coupled charge interaction of a multi-layer structure. This structure is a three-ply stack of a ferromagnetic material (FM), a non-magnetic conductive layer (NM) and a bottom layer of ferromagnetic material (FM) all on a silicon substrate.
One of the challenges with XMR sensors is that they have no means of discriminating magnetic fields sensed along the easy axis. Because the level of the field of interest will be orders of magnitude lower than the background drive magnetic field, it has been historic precedent to either: (a) shield the sensor from the drive field or (b) orient the sensor such that the sensitive axis is orthogonal to the drive field as to not saturate the sensing element. For example, one can find the use of concentric/co-located sensors and drives in U.S. patents: U.S. Pat. No. 6,888,346, 2011/0068784 A1, 2005/0007108 A1, U.S. Pat. No. 6,888,346 as well as 2005/0007108 A1. All of the documents cited herein are incorporated by reference in their entireties. This has led to the vast majority of embodied XMR based ECT probes towards using the sensor in a horizontal sensing configuration with respect to the AUT while positioned in the center of an excitation coil. As illustrated in FIG. 1, this allows the sensor 10 to be co-located with the drive coil 20 and positioned in the bore of the coil orienting the excitation field BEX orthogonally to the sensor easy axis. Therefore the sensor would be immune to the excitation field.
This configuration of XMR 10 to the surface of the AUT 30 does not lead to the same signal morphology in response to a material defect 31 as a wound pancake inductive coil ECT probe. Signal morphology is a critical ECT product requirement as there is often continuity required with historic inspection data. A pancake coil based ECT probe does a spatial integration of the time rate of change of all three axes of magnetic flux leakage at any point in space created by perturbation of the eddy current distribution by a discontinuity in the material. Because the largest vector component BZMFLof the of the out-of-plane MFL will be the component orthogonal to the AUT, it is the most dominant component in the coils' spatial integration and thus influences most the eddy current signal response. This is best approximated by vertical sensing methodologies (in Cartesian coordinates or radial in cylindrical coordinates) that align the easy axis of the XMR sensor with this field component of the MFL as shown in FIG. 2.
Because of the aforementioned reasons, to date, there has not been a practicable ECT probe that orients an XMR sensor with the easy axis aligned orthogonal to the surface of the AUT.